NUCLEUS is a base theme first released with Drupal 7 by ThemeBrain. The goal of this base theme is to provide a solid foundation for web development geeks to develop a theme quick and easy. Having a fast learning curve is the inflection point for UX/UI designers, Back-End and Front-End Developers.

The Breakpoints module allows you to group breakpoints together. This way other modules can use these groups. The Picture module, for example, allows you to select a breakpoint group when you select the Picture display formatter for a field. The formatter will determine the size of the image to display based on the breakpoints in the selected group.

Following these steps you can create your own groups:

Navigate to the breakpoints configurations page

Go to Configuration » Media » Breakpoints. You will notice you can add your own breakpoints on this page.
In stead, click on "Add a new group":

Configure the breakpoints group

Enter a name for the new breakpoints group and select the breakpoints to include in this group:

Save the form and your new group should show up at the top of the page. You should also see the breakpoints selected on the previous step in this group: